The present invention relates to frequency mistuned blade rows for use in turbomachinery such as gas turbine engines, steam engines, and the like. More particularly, the invention relates to an array of flow directing elements to be used as the rotor blades of multi-stage fans without shrouds that have improved flutter resistance and decreased weight.
Many aircraft jet engines employ the turbofan cycle, in which a fan section of turbomachinery is used to both generate thrust and supply compressed air to the core of the engine. The fan section of engines for commercial applications typically consists of an isolated rotor, whereas the fan section of engines for military applications typically consists of multiple stages of blades and vanes. The latter configuration is referred to as a multi-stage fan. It faces more challenging aerodynamic conditions than an isolated fan due to the interaction of the fan stages. One consequence of this interaction is that the first stage rotor of a multi-stage fan has stricter design restrictions for avoiding flutter than an isolated fan.
Axial flow turbomachinery blades are subject to destructive vibrations due to unsteady interaction of the blades with the working fluid. These vibrations are generally categorized as forced response or flutter. Both categories of vibrations can cause structural failure of the turbomachinery blades.
The turbomachinery blades have natural vibration modes of increasing frequency and complexity of the mode shape. The simplest and lowest frequency modes are typically first bending (1B), second bending (2B), and first torsion (1T). First bending is a motion normal to the flat surface of the airfoil portion of the blade in which the entire span of the airfoil moves in the same direction. Second bending is similar to first bending, but with a change in the sense of the motion somewhere along the span of the airfoil so that the upper and lower portions of the airfoil move in opposite directions. First torsion is a twisting motion around an elastic axis, which is parallel to the span of the airfoil, in which the entire span of the airfoil on each side of the elastic axis moves in the same direction.
Forced response vibration typically occurs when an integral multiple of an engine""s rotation frequency, known as an engine order excitation, coincides with one of the natural vibration frequencies of the flow directing elements or blades. When these frequency coincidences occur, the flow directing elements or blades will vibrate in resonance. This can cause vibrations of sufficient amplitude to cause structural failure. These frequency coincidences are typically avoided by tuning the blades"" natural vibration frequencies to avoid engine order excitations over rotational speed ranges where the engine spends a significant portion of its operating cycle.
Engine order excitations are characterized as multiples of the engine rotation frequency, so that a xe2x80x9c1Exe2x80x9d excitation is at the engine rotation frequency, a xe2x80x9c2Exe2x80x9d excitation is at twice the engine rotation frequency, etc. Conventional tuning criteria for unshrouded blades in the first stage of a multi-stage fan is for the 1B frequency to be above the 2E excitation frequency, and that all lower order modes, typically the four lowest frequency modes, avoid engine order excitation frequencies in the operating range. Another criterion is that the 1B frequency does not match the 2E or 3E excitation frequencies at idle operating conditions.
Flutter is an aero-elastic instability resulting from interaction of the flow over the flow directing elements or blades and the blades"" natural vibration tendencies. When flutter occurs, the unsteady aerodynamic forces on a flow directing element due to its vibration add energy to the vibration, thus causing the vibration amplitude to increase. The vibration amplitude can become large enough to cause structural failure. The operable range, in terms of pressure rise and flow rate, of the engine is restricted by various flutter phenomena.
Lower frequency vibration modes, first bending and first torsion, are the vibration modes that are typically susceptible to flutter. Conventional practice to avoid flutter is to raise the blades"" first bending and first torsion vibration frequencies, and/or increase the blades"" chord length and/or add a shroud to provide mechanical contact between adjacent airfoils. Thus, conventional design practices to avoid flutter add length and weight to rotor blades that is not required for aerodynamic performance, and the use of thicker blades or shrouds imposes an aerodynamic performance penalty. If flutter design restrictions are relaxed, then lighter and shorter blades can be employed, and the length and weight of the turbomachinery is reduced. Lighter parts provide obvious benefits for the turbomachinery of aircraft jet engines.
Conventional practice for unshrouded blades in the first stage of multi-stage fans is to tune the blades so that the first bending frequency is above the second harmonic of the rotation frequency. This tuning practice avoids forced response vibrations while resulting in vibration frequencies that are typically high enough to avoid flutter. Relaxing flutter design restrictions would allow the blades to be tuned so that the first bending frequency is above the first harmonic of the rotation frequency and below the second harmonic in the operating range. Since the frequency of the first bending mode is directly proportional to the thickness of the blade at the root, tuning blades to a lower frequency results in thinner blades that reduce weight and improve performance.
Blades are more susceptible to flutter instability if all blades on a rotor disk have nearly identical vibration frequencies. Advances in manufacturing technique have resulted in the production of blades that have nearly uniform properties. This uniformity is desirable to ensure consistent aerodynamic performance, but undesirable in that it increases the blades"" tendency to flutter. Therefore, to ensure that a minimum level of nonuniformity of the blades is achieved, it is desirable to introduce intentional variation to mistune the blades and thus achieve flutter resistance.
These intentional variations should significantly affect the vibration frequency of the blade without compromising aerodynamic performance or introducing undue complexity in the manufacturing process. One method of achieving the frequency variation between blades is to vary the thickness of individual blades around the rotor. Other methods include, but are not limited to, variations in chord, camber angle, and profile shape. Variations of blade geometry in the inner span region, where the flow tends to be subsonic, tend to introduce less aerodynamic performance variation than geometric variations in the outer span region, where the flow tends to be supersonic.
Flutter resistance increases as the difference in frequency between blades increases, up to a theoretical maximum. Manufacturing tolerances introduce frequency variations that are typically +/xe2x88x923% of the nominal frequency. However, modern manufacturing techniques can result in frequency variations that are less than 1%, which can reduce flutter resistance. Thus, for manufacturing processes which result in a relatively small variation in frequency, intentional mistuning can increase flutter resistance.
The use of nonuniformity in vibration frequency to avoid flutter instability for turbomachinery blades is addressed in U.S. Pat. No. 5,286,168 to Smith. The approach discussed therein uses frequency mistuning for flutter avoidance, but does not use the reduced flutter susceptibility to alter blade tuning criteria and thus lower blade weight.
The use of nonuniformity in shroud angle to avoid flutter instability for a blade row of shrouded blades is addressed in U.S. Pat. No. 5,667,361 to Yaeger et al. The approach discussed in the Yaeger et al. patent however is unattractive for modern gas turbine engines since the use of shrouds imposes an aerodynamic performance penalty.
Accordingly, it is an object of the present invention to provide an improved array of flow directing elements for use in turbomachinery devices.
It is a further object of the present invention to provide an improved array as above which is mistuned to increase flutter stability.
It is yet a further object of the present invention to provide an array as above which is relatively light in weight and does not disturb aerodynamic performance.
The foregoing objects are attained by the array of the present invention.
An improved array of flow directing elements for use in turbomachinery devices, such as gas turbine engines, steam engines, and the like is provided by the present invention. The improved array broadly comprises a row of alternating high frequency and low frequency flow directing elements or blades. The high frequency flow directing elements each have their three lowest frequency vibratory modes at least 2.0% higher in frequency than the three lowest frequency vibratory modes of each low frequency flow directing element. The array of the present invention may be used in turbomachinery as a blade row for a rotor stage.
Other details of the array of flow directing elements, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.